1. Field of Invention
This invention relates to tubular shaft members and in particular, relates to tubular members having high stiffness to weight ratios which are ideally suited for power transmission in aircraft such as the drive shaft for a helicopter tail rotor or a remotely mounted propeller.
2. Description of the Prior Art
A number of aircraft have been developed which require the transmission of power from an aircraft engine to a remotely driven member such as the tail rotor of an aircraft or a remotely mounted propeller driven from a midship mounted engine. In such installations it is necessary to achieve minimum weight of the shaft and bearing supports while maintaining sufficient strength and stiffness in the tubular drive member to resist torsional and lateral oscillations. The problem is particularly acute in drive shafts for the helicopter tail rotors which are separated from the drive engine by a substantial length of fuselage which flexes during normal maneuvers of the helicopter or in helicopters having tandem, dual rotors which are driven through drive shafts which extend from a centrally located engine station.
Heretofore, the drive shafts employed for such aircraft installations have been fabricated of tubular metal members which bear radial end flanges for their interconnections to each other and to the engine drive or working member such as the tail rotor. Typically, the end flanges are approximately the same diameter as the tubular section of the member and are carried at the ends of the frustoconical sections which provide access to the radial faces of the flanges for mounting interconnecting fasteners between adjoining flanges. The radial end flanges are connected at bearing stations to a flexible coupling that generally comprises a series of stacked plates which permit some flexing between the coupling member and the tubular shaft member. The bearing stations are required to permit a limited amount of flexing of the tubular drive shaft to accommodate flexing of the supporting fuselage and to permit joining of the tubular sections which are limited in length to avoid development of resonating oscillations at critical natural frequencies in the tubular shaft members. It is desirable to minimize the number of the bearing stations in any installation because of the associated weight of the components of the bearing station. Since most metals have approximately equal stiffness to weight ratios, the distance between bearing stations for metalic tubular members is generally only a function of the diameter of the tubular members and is independent of the metal identity. It is, therefore, desirable to employ tubular members as drive shaft components which are formed of materials having stiffness to weight ratios superior to those of metallic members.
With many helicopter applications, particularly for military aircraft, the metallic drive shaft members are susceptible to catastrophic failure from relatively minor damage. Metallic members, when subjected to torsional stress, are highly notch sensitive so that if struck by a bullet or other fragment, the entire member will fail, resulting in loss of the aircraft. It is therefore desirable to provide a drive shaft formed of a material which is less stress tear sensitive than metallic members.